1. Field of the Invention
The invention relates to hydraulic pressure control apparatus and method for a vehicular automatic transmission. More particularly, the invention relates to improving failsafe technology which enables the vehicle to run appropriately in the event that transmission state changing means for changing a power transmission state of the automatic transmission fails.
2. Description of the Related Art
A hydraulic pressure control apparatus for a vehicular automatic transmission is known which is provided with a plurality of hydraulic friction engaging devices that are selectively operated for selectively establishing a speed of the automatic transmission, a plurality of electromagnetic control valve devices which change the combination of the plurality of hydraulic friction engaging devices being operated so as to establish an arbitrary speed, and a driving state switching valve that is switched by an operation by a driver so as to switch the driving state of the automatic transmission by supplying hydraulic fluid fed from a hydraulic pump under pressure to a predetermined hydraulic fluid path. With this hydraulic pressure control apparatus, multiple speed shifting in, for example, six forward speeds is possible by the plurality of electromagnetic control valve (i.e., shift valve) devices each controlled by a separate solenoid, for example. Normally automatic transmissions are provided with a predetermined electronic control unit (ECU) that electrically controls the plurality of individual solenoids and the like. The ECU, the plurality of individual solenoids, and the plurality of electromagnetic control valve devices and the like constitute the transmission state changing means.
It is conceivable, however, that this transmission state changing means may, for one reason or another, stop working, or in other words, fail. In view of this possibility of failure, it is necessary to introduce failsafe technology into the design of the apparatus. Particularly, it is necessary to have the vehicle be able to continue driving if a failure occurs while driving. These needs have led to the proposal of failsafe means for enabling a vehicle to run appropriately even if the transmission state changing means fails. One such proposal is a hydraulic pressure control apparatus for an automatic transmission disclosed in Japanese Patent Laid-Open Publication No. 2000-170899. The hydraulic pressure control apparatus of that publication is provided with a plurality of friction engaging elements, a plurality of hydraulic servos that engage and disengage those friction engaging elements according to supply or drain of hydraulic fluid, a solenoid valve which generates a predetermined hydraulic pressure and supplies that predetermined hydraulic pressure to one or more of the hydraulic servos selected in accordance with a speed to be established, and failsafe means which supplies hydraulic pressure for the failsafe to at least two of the selected hydraulic servos to establish an arbitrary speed when a failure has occurred in the solenoid valve such that power is unable to be transmitted. As a result, in the event that a failure occurs while driving, at least two hydraulic servos are selected to establish an arbitrary speed. Hydraulic pressure for the failsafe is then supplied to each of these hydraulic servos so as to enable the vehicle to continue running.
However, a vehicle not provided with a low speed range, i.e., “3,” “2” or “L”, such as a sequential shift vehicle (i.e., a type of vehicle that runs forward only when in the “D” range where shifting is performed by electric means), may have not be able to obtain the necessary driving force while in a high speed and may therefore have difficulty taking off when attempting to take off again after stopping the vehicle when the failsafe means is functioning.